Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
  • G01N27/04—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance
  • G01N27/12—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance of a solid body in dependence upon absorption of a fluid; of a solid body in dependence upon reaction with a fluid, for detecting components in the fluid
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
  • G01N33/0004—Gaseous mixtures, e.g. polluted air
  • G01N33/0009—General constructional details of gas analysers, e.g. portable test equipment
  • G01N33/0027—General constructional details of gas analysers, e.g. portable test equipment concerning the detector
  • G01N33/0036—General constructional details of gas analysers, e.g. portable test equipment concerning the detector specially adapted to detect a particular component
  • G01N33/004—CO or CO2

Definitions

• the invention relates to a sensor for the determination of carbon monoxide, in particular in gas mixtures containing oxygen, based on doped metal oxides, the electrical conductivity or electrical resistance of which is a function of the composition of the gas mixtures.
• Carbon monoxide sensors are e.g. for monitoring heating systems based on fossil fuels or internal combustion engines. In both cases, economic and ecological
• DE-PS 26 03 785 describes sensors made of chrora (III) oxide or tin (IV) oxide which are combined with at least one oxide of the gear metals of the 4th to 6th group of the periodic table or an oxide of iron, nickel, cobalt, tin, magnesium, calcium or lithium is doped.
• sensors with semiconductors consist of doped tin (IV) oxide, the catalytic activity of the semiconductor for the reaction between oxygen and oxidizable components, such as carbon monocide, being specifically reduced.
• Sensors based on cerium (IV) oxide which is doped with magnesium oxide, aluminum oxide, yttrium oxide, titanium (IV) oxide, tantalum (V) oxide, niobium (V) oxide or vanadium (V) oxide described in DE-PS 3024449.
• the sensors according to the invention show a lower temperature dependence of the conductivity than the known sensors. This applies in particular if they are additionally doped with tantalum (V) oxide. They therefore lead to good and reproducible measurement results even without complex compensation circuits, which are required in the case of sensors according to the prior art when there are strong temperature fluctuations in the exhaust gases.
• Another advantage of the sensors according to the invention lies in their extended service life. Downtimes of 5000 and more operating hours can be achieved.
• the sensors according to the invention are particularly suitable for the determination of carbon monoxide in poor, i.e. low carbon monoxide and comparatively high oxygen-containing exhaust gases.
• the levels of carbon monoxide can e.g. 5 to 1500 ppm, preferably 10 to 1200 ppm, the oxygen 2 to 20 vol%, preferably 2 to 10 vol%.
• the predominant proportions are nitrogen and carbon dioxide, but smaller amounts of nitrogen oxides and / or unburned substances can also be present in the mixture Contain hydrocarbons.
• the components of the sensors are related to doped
• metal oxides which have an n-conductivity are those which have a low oxygen deficit compared to the stoichiometry. Examples include zinc oxide and cerium (IV) oxide. Tin (IV) oxide has proven to be very suitable.
• oxides of other metals with a maximum valence ⁇ 4 aluminum oxide, copper (I ⁇ ) oxide, iron (II) oxide, iron (III) oxide, nickel oxide, cobalt oxide, calcium oxide and strontium oxide may be mentioned, for example.
• Magnesium oxide is preferably used.
• the metal oxides are added only in small amounts, in particular in amounts of 0.05 to 0.15 mol%.
• the type and amount of catalyst is selected.
• Suitable oxides of metals which catalyze the reaction of carbon monoxide with oxygen to form carbon dioxide are, among others, suitable for sensors according to the invention. those of the platinum metals platinum, rhodium and ruthenium and in particular palladium; the oxides of manganese, chromium, cobalt and egg disks can also be used.
• n-conducting metal oxide additionally contains an oxide of a metal of the fifth group of the periodic table of the elements, ie vanadium, niobium and in particular tantalum, have a particularly small resistance-temperature coefficient.
• This additive is also used in only small amounts, advantageously 0.003 to 0.03 mol%. If the two dopants according to claim 1 and the additive according to claim 2 are referred to as oxides, then this should only refer to the degree of oxidation of the metal in the manufacture of the sensor.
• the metals in question may be present in the finished sensors at least partially in the form of other chemical compounds, for example as stannates, or in metallic form, for example as palladium.
• the doped n-type metal oxides are made in the usual way
• the dry doped n-conducting metal oxide is then, if appropriate after annealing at 500 to 650 ° C., expediently ground in fine particles, pasted and applied to a support provided with electrodes, for example made of platinum.
• the coated carrier is allowed to dry and the doped n-type metal oxide is activated by gradually heating it on the carrier to a temperature of about 800 to 1000 ° C and also gradually cooling it again.
• the sensor obtained in this way can be connected directly into the measuring circuit as a resistor.
• the solid mass is slurried with 3 liters of water, digested for 4 hours and the water decanted. This treatment is repeated with 4 liters of water.
• the coarsely crystalline mass is heated to 600 ° C. in a tube furnace at a temperature increase from 50 ° C./hour and kept at this temperature for 5 hours. The mass is left with a cooling rate of
• the dry powder is mixed with a "thick oil" (50% benzyl alcohol, 30% ethyl cellulose, 20% terpineol); the volume ratio of powder to paste is approximately 1: 1.
• a "thick oil” 50% benzyl alcohol, 30% ethyl cellulose, 20% terpineol
• the volume ratio of powder to paste is approximately 1: 1.
• the paste is spread on a ceramic support provided on the top with two platinum electrodes and on the underside with a printed platinum heating meander so that the layer, approximately 0 , 1 mm thick, appears even.
• the coated support is allowed to air at 70 ° C for about 10 minutes and is then gradually heated (temperature increase about 20 ° C / h) to 900 ° C. After this temperature has been maintained for 2 hours, the sensor is allowed to cool to room temperature with a temperature drop of approximately 100 ° C./h.
• Fig. 1 to 3 show the resistance-temperature characteristics of various sensors in logarithmic plots.
• Fig. 1 relates to a sensor according to the prior art
• Fig. 2 shows the properties of a sensor according to claim 1
• Fig. 3 shows those of a sensor according to claim 2.
• the doped n-type metal oxides have the following compositions: Fig. 1: Tin (IV) oxide with 4 mol% magnesium oxide, 2 mol% aluminum oxide, 3 mol% copper (II) oxide
• Fig. 2 Tin (IV) oxide with 0.10 mol% magnesium oxide, 0.14 moli palladium oxide
• Fig. 3 Tin (IV) oxide with 0.10 mol% magnesium oxide, 0.10 mol% palladium oxide, 0.005 mol% tautal (V) oxide
• the third curve shows measurements with the gas mixture of the first curve.
• the sensors according to the invention show no appreciable hysteresis, but do show a high sensitivity to carbon monoxide and, in particular, a markedly lower dependence of the resistance or the conductivity on the temperature, as measured by the prior art. This applies particularly to the sensor according to Fig. 3.

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• Chemical & Material Sciences (AREA)
• Health & Medical Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• General Health & Medical Sciences (AREA)
• Immunology (AREA)
• Pathology (AREA)
• Analytical Chemistry (AREA)
• Biochemistry (AREA)
• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Combustion & Propulsion (AREA)
• Food Science & Technology (AREA)
• Medicinal Chemistry (AREA)
• Investigating Or Analyzing Materials By The Use Of Fluid Adsorption Or Reactions (AREA)

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• 1991
  • 1991-01-15 DE DE4100915A patent/DE4100915A1/de active Granted
  • 1991-12-18 DE DE59108090T patent/DE59108090D1/de not_active Expired - Fee Related
  • 1991-12-18 US US08/078,291 patent/US5351029A/en not_active Expired - Lifetime
  • 1991-12-18 JP JP4501630A patent/JP3053865B2/ja not_active Expired - Lifetime
  • 1991-12-18 EP EP92900789A patent/EP0567464B1/de not_active Expired - Lifetime
  • 1991-12-18 ES ES92900789T patent/ES2091454T3/es not_active Expired - Lifetime
  • 1991-12-18 KR KR1019930702100A patent/KR100189041B1/ko not_active Expired - Fee Related
  • 1991-12-18 WO PCT/DE1991/000986 patent/WO1992013270A1/de not_active Ceased
  • 1991-12-18 AU AU90514/91A patent/AU9051491A/en not_active Abandoned

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